Temperature-dependent molecular sieving of fluorinated propane/propylene mixtures by a flexible-robust metal-organic framework

The electronics industry necessitates highly selective adsorption separation of hexafluoropropylene (C3F6) from perfluoropropane (C3F8), which poses a challenge due to their similar physiochemical properties. In this work, we present a microporous flexible-robust metal-organic framework (Ca-tcpb) with thermoregulatory gate opening, a rare phenomenon that allows tunable sieving of C3F8/C3F6. Remarkably, the temperature-dependent adsorption behavior enhances the discrimination between the larger C3F8 and the smaller C3F6, resulting in unprecedented C3F6/C3F8 selectivity (over 10,000) compared to other well-known porous materials at an optimal temperature (298 K). Dynamic breakthrough experiments demonstrate that high-purity C3F8 (over 99.999%) could be obtained from a C3F6/C3F8 (10:90) mixture under ambient conditions. The unique attributes of this material encompass exceptional adsorption selectivity, remarkable structural stability, and outstanding separation performance, positioning it as a highly promising candidate for C3F6/C3F8 separation. Single-crystal structural analysis of C3F6-loaded Ca-tcpb and theoretical calculations elucidate the host-guest interaction via multiple intermolecular interactions.


Fitting of pure component isotherms
The experimentally measured loadings for C3F6 and C3F8 measured at temperatures of 298 K in selected material were fitted with the Dual-Langmuir-Freundlich isotherm model.
where qA,sat and qB,sat (mol•kg -1 ) are the saturated capacities of sites A and B, respectively, bA and bB (1/kPa) are the affinity coefficients to the sites A and B, respectively, p (kPa) is the pressure of the bulk gas at equilibrium with the adsorbed phase (kPa), q (mol•kg -1 ) is the gas uptake amount of an adsorbent, and nA and nB represent the deviations from an ideal homogeneous surface.

IAST calculations of adsorption selectivity
In order to compare the C3F6/C3F8 separation potential of various porous materials, IAST calculations of mixture adsorption (10/90) were performed.For separation of a binary mixture of components i and j, the selectivity coefficient (Sij) has been defined as: Where, and refer to the equilibrated adsorption capacity of component i and j, and refer to the molar fraction of component i and j in gas phase.

Fig. S8 .
Fig. S8.N2 sorption isotherms of 1a at 77 K. (A) and (C): Solid and open symbols represent adsorption and desorption branches, respectively.The x-axis is presented in logarithm in (C).(B) The calculated BET surface area of 1a.

Fig. S17 .
Fig. S17.The studies on the variation of cell parameters.(A) Cell length/angle of 1a at different temperatures.The b-axes changes linearly by +0.03% and the beta angle changes by + 0.1% in the temperature range of 273-313 K; and (B) Comparison of cell parameters before and after loading C3F6.The b-axes and the beta angle show an increase of + 2.8%, + 1.1% on C3F6@1a, respectively.

Fig. S23 .
Fig. S23.In-situ C3F8-loaded PXRD.No. letters in adsorption correspond to the numbering of the PXRD patterns: (A) and (E) Adsorption isotherm at 283 K and 298 K. (B) and (F) PXRD pattern recorded during loading C3F8 under different pressures at 283 K and 298 K. (c) and (g) of the PXRD pattern evolution of C3F8 at 283 K and 298 K. (D) and (H) Partially enlarged contour plot.

Fig. S26 .
Fig. S26.The overlay diagram of the variable temperature single crystals of 1a.

Table S1 . Reference to synthesis or vendor of the materials.
a Used without further purification